The present invention concerns a method of checking the living character of a finger by means of a fingerprint sensor. The invention also confirm the fingerprint sensor for implementing this method.
In general terms, any protected access becomes accessible to an authorized person by a means that he alone possesses. One of the means of limiting access to a person is to require the fingerprint of this person. The image of the fingerprint of a person is obtained by a fingerprint sensor. Once the image of the print is obtained by the sensor, it is transmitted to an image processing unit that compares the image obtained with a bank of print images so as to check that the print taken by the sensor is known. Recognition of the print by the image processing unit then opens up, to the person to whom the print corresponds, access to that which he seeks.
It has been found that, although identification by fingerprints is a known method, it still poses problems. This is because there are many forgers who attempt to deceive fingerprint sensors with imitations. The artifices in particular used are false fingers.
In order to thwart such forgers, several methods have been proposed for determining whether the element carrying the fingerprint is living. Certain methods use optical means. This is for example the case with the document U.S. Pat. No. 5,719,950, which describes a method consisting of measuring biometric parameters such as the oxygen level in the blood, the temperature of the skin, etc. The document U.S. Pat. No. 5,737,439 describes an optical measurement system for detecting blood flow by means of two wavelengths. Other methods consist of making electrical measurements. This is the case with the document JP-A-11197135, which describes the measurement of variations in capacitance between two electrodes, and the document U.S. Pat. No. 5,953,441, which describes a device for measuring the complex impedance of the finger and comparing it with reference curves which are a function of frequency.
It has been found through the measures already known that the measurement of the impedance of the finger is one of the methods best suited to checking the living character of a finger. The technique consists of measuring an impedance Z between two electrodes and comparing the value obtained by this measurement with a range of values considered to be acceptable. The technique is still sometimes deceived by imitations and discrimination errors remain numerous.